In normal human hearing, acoustical energy in the form of sound waves is directed into the ear canal of a human by an outer ear. The sound waves impinge upon a tympanic membrane, i.e. the eardrum, located at the inner end of an outer ear canal. The pressure of the sound waves causes tympanic vibrations in the eardrum, thereby producing mechanical energy.
Three interconnected bones, referred to as the ossicular chain, transfer these tympanic vibrations of the eardrum across a middle ear cavity and into an inner ear. The ossicular chain includes three major bones, the malleus, the incus and the stapes. The stapes resides in the oval window, attached to its margins by the annular ligament. The oval window serves as the entrance to the inner ear.
Mechanical vibrations conducted to the oval window generate vibrations within the inner ear fluids, the perilymph and then the endolymph. The hearing portion of the inner ear is a hollow, spiral otic capsule bone shaped like a snail shell and called the cochlea. The cochlea is divided into three chambers, the scala vestibuli, scala tympani which contain perilymph, and the scala media which contains endolymph. Sound vibrations (pressure waves) enter the perilymph of scala vestibuli and are transmitted to scala media across a thin elastic membrane (Reisner's membrane). The floor of scala media is the basilar membrane, a flexible membrane which has an elasticity gradient progressing from stiff to flexible. The varying resonant characteristics of the basilar membrane permit pitch differentiation with the basal coil of the cochlea being sensitive to high frequencies and the apical to low frequencies. Positioned on the basilar membrane are 16,000 receptor cells (“hair cells”) arranged in three rows of outer hair cells, and one row of inner hair cells. The cilia of these hair cells insert into a rigid tectorial membrane. As the basilar membrane is displaced upward the cilia bend, the shearing effect produces a change in membrane permeability of the hair cells and potassium contained in the potassium rich endolymph invades the hair cells, depolarizing the cell. The bases of the hair cells are innervated by auditory nerve fibers which are activated by this depolarization. The auditory nerve fibers then transmit signals ultimately to the temporal lobe of the brain where the subject consciously perceives sound.
Generally, hearing difficulties fall into one of two categories. Conductive hearing loss relates to the inability, or inefficiency, in mechanically conveying the vibrations caused by sound waves through the outer ear, the middle ear and the oval window to the perilymph. Sensorineural hearing impairment relates to deterioration of the receptor cells or nerve fibers within the inner ear, so that fluid vibrations within the inner ear are not properly converted to nerve impulses and thus inadequately transmitted to the brain.
Hearing loss is typically treated using two different types of hearing devices. The most common category of device is the hearing aid. For more significant hearing impairments, cochlear implants are employed.
Hearing aids simply provide a form (via analog or digital processing) of acoustical gain to improve the hearing of patients with hearing loss. There are a variety of forms of hearing aids to account for preferences of the various patients and their respective degrees of hearing loss. A number of hearing aids include a “behind-the-ear” component and an “in-the-ear” component. Relatively mild hearing loss can be addressed using “in-the-ear” components that do not completely seal the ear canal. However, to achieve greater acoustical gain for patients with greater hearing loss, other known in-the-ear components seal the ear canal. Some patients can find devices that seal the ear canal to be uncomfortable or undesirable in some respects. Due to a number of issues, many patients suffering from hearing loss choose not to use their hearing aids for a significant amount of time or at all.
Instead of merely amplifying acoustical signals, cochlear implants process acoustical signals and generate electrical currents for stimulation of neural tissue of the cochlea. Typically, a cochlear system will include a behind-the-ear component that samples acoustical signals and processes the sampled signals using a digital signal processor and various processing algorithms. Cochlear systems also include multicurrent sources for generating stimulation currents corresponding to various frequency bands of the processed signals. The multi-stimulation currents are applied to different portions of the cochlea by respective electrodes of a cochlear lead. Also, it is noted that implantation of the cochlear lead is a relatively delicate process. Due to these considerations, cochlear implants are only utilized for patients with relatively profound hearing losses.